1. Field of the Invention
The present invention relates to a piezoelectric electroacoustic transducer such as a piezoelectric buzzer and piezoelectric receiver, and a manufacturing method thereof.
2. Description of the Related Art
In previous electronic devices, such as electronic home appliances and portable telephones, piezoelectric electroacoustic transducers have been used as piezoelectric buzzers or piezoelectric receivers for producing an alarm sound or operating sound. A conventional piezoelectric electroacoustic transducer includes a circular piezoelectric element that is bonded on one surface of a circular metallic plate to define a unimorph-type diaphragm, the periphery of the metallic plate is supported with silicone rubber in a circular case, and an opening of the case is closed with a cover.
However, with a circular diaphragm, production efficiency and acoustic conversion efficiency is relatively low, and the size of the piezoelectric electroacoustic transducer cannot be sufficiently reduced.
A piezoelectric electroacoustic transducer that includes a square diaphragm and achieves improved production efficiency, improved acoustic conversion efficiency and a reduced size, is disclosed in Japanese Unexamined Patent Application Publication No. 2000-310990. The piezoelectric electroacoustic transducer includes a square piezoelectric diaphragm, an insulating case having a bottom wall, four sidewalls, and a supporting member disposed inside two of the sidewalls opposing each other for supporting the diaphragm, the supporting member being provided with first and second conductive portions for external connection, and a lid plate having holes provided therein for releasing sound. The diaphragm is accommodated within the case such that two opposing sides of the diaphragm are fixed to the supporting member with an adhesive or an elastic sealant, while gaps between the remaining two sides of the diaphragm and the case are sealed with the elastic sealant, such that the diaphragm and the first and second conductive portions are electrically connected together via a conductive adhesive, and the lid plate is bonded on the open end of the side-wall of the case.
In recent years, the diaphragm thickness of the piezoelectric electroacoustic transducer has been greatly reduced, and a diaphragm having a thickness of approximately several tens to 100 xcexcm is used. When using such a reduced-thickness diaphragm, the structure for supporting the diaphragm greatly influences the frequency characteristics.
For example, when directly connecting the diaphragm to an external electrode with a thermosetting conductive adhesive, such as urethane, a strain is produced in the diaphragm due to the curing and contraction stress of the conductive adhesive, such that the frequency characteristics are changed. Also, when the ambient temperature changes, the characteristics change due to the difference in thermal expansion coefficients of the case and the diaphragm, and when an external force is directly applied to the case, the force is also directly transmitted to the diaphragm, such that the characteristics change.
After fixing the diaphragm to the case with an elastic supporting material as described above, even when coating the diaphragm and case with the conductive adhesive, if the shortest distance between the two sides opposing each other of the diaphragm and a supporting member of the case, i.e., an internal connection portion for electrical connection to an external electrode, is coated with the conductive adhesive, a stress caused by the curing and contraction of the conductive adhesive is applied to the diaphragm, such that variations in the frequency characteristics are produced.
In order to overcome the above-described problems, preferred embodiments of the present invention provide a piezoelectric electroacoustic transducer and a manufacturing method thereof, in which stability of the frequency characteristics is achieved by selectively coating locations of an adhesive for fixing a diaphragm to a case and of a conductive adhesive for electrical connection, so as to prevent stress being imparted to the diaphragm.
A first preferred embodiment of the present invention provides a piezoelectric electroacoustic transducer which includes a substantially square piezoelectric diaphragm having electrodes and flexurally vibrating in a plate-thickness direction in response to application of an alternating signal between the electrodes, a substantially rectangular insulating case having a supporting member disposed inside sidewalls of the case for supporting the piezoelectric diaphragm, terminal electrodes having internal connections exposed in the vicinity of the supporting member and external connections exposed on the external surface of the case and electrically connected to the internal connections, a first adhesive which is applied to a shortest route connecting the piezoelectric diaphragm to the internal connections, the shortest route being located between the external periphery of the piezoelectric diaphragm and the internal connections, so as to fix the piezoelectric diaphragm to the case, a conductive adhesive for electrically connecting the electrodes of the piezoelectric diaphragm to the internal connections of the terminal electrodes, the conductive adhesive being applied between the electrodes of the piezoelectric diaphragm and the internal connections via the upper surface of the first adhesive by detouring from the shortest connection route between the piezoelectric diaphragm and the internal connections, and a second adhesive for sealing a gap between the external periphery of the piezoelectric diaphragm and the internal periphery of the case, wherein the first and second adhesives have a Young""s modulus after curing less than that of the conductive adhesive.
A second preferred embodiment of the present invention provides a method for manufacturing a piezoelectric electroacoustic transducer, which includes the steps of preparing a substantially square piezoelectric diaphragm flexurally vibrating in a plate-thickness direction in response to application of an alternating signal between electrodes, preparing a substantially rectangular insulating case having a supporting member disposed inside sidewalls of the case for supporting the piezoelectric diaphragm and terminal electrodes having internal connections exposed in a vicinity of the supporting member and external connections exposed outside the case and electrically connected to the internal connections, applying a first adhesive to a shortest route connecting the piezoelectric diaphragm to the internal connections, the shortest route being located between the external periphery of the piezoelectric diaphragm and the internal connections, so as to fix the piezoelectric diaphragm to the case by curing the first adhesive, applying a conductive adhesive between the electrodes of the piezoelectric diaphragm and the internal connections via the upper surface of the first adhesive by detouring from the shortest connection route between the piezoelectric diaphragm and the internal connections so as to electrically connect the electrodes of the piezoelectric diaphragm to the internal connections of the terminal electrodes by curing the conductive adhesive, and applying a second adhesive in a gap between the external periphery of the piezoelectric diaphragm and the internal periphery of the case so as to seal both of the peripheries by curing the second adhesive, wherein the first and second adhesives have a Young""s modulus after curing that is less than that of the conductive adhesive.
According to preferred embodiments of the present invention, after fixing the external periphery of the diaphragm and the internal connections of the terminal electrodes with the first adhesives, the electrodes of the piezoelectric diaphragm and the internal connections of the terminal electrodes are electrically connected together with the conductive adhesives. At this time, the first adhesive is applied and cured to the shortest routes between the piezoelectric diaphragm and the internal connections, which are located between the external periphery of the piezoelectric diaphragm and the internal connections, while the conductive adhesive is applied and cured thereon via the upper surfaces of the first adhesives by detouring from the shortest route between the piezoelectric diaphragm and the internal connections. Since the first adhesive has a Young""s modulus after curing that is less than that of the conductive adhesive, the stress caused by the curing and contraction of the conductive adhesive is relieved by the first adhesive, such that the stress is not directly applied to the piezoelectric diaphragm. Therefore, the strain is not produced in the piezoelectric diaphragm which eliminates variations in frequency characteristics. Even when the ambient temperature is changed or an external force is applied to the case, the stress is relieved by the first adhesive, such that the stress does not affect the piezoelectric diaphragm, thus preventing frequency characteristics from varying.
When manufacturing the piezoelectric electroacoustic transducer according to preferred embodiments of the present invention, the first adhesive may be applied after the piezoelectric diaphragm is provided within the case, or before the piezoelectric diaphragm is provided within the case, the first adhesive may be applied to the external periphery of the piezoelectric diaphragm or in the vicinity of the supporting member of the case. In the former case, the first adhesive is applied preferably using a dispenser, whereas in the latter case, the first adhesive is applied preferably using not only the dispenser but also a trowel, the first adhesive may be applied to end portions of the piezoelectric diaphragm so as to bond and fix the piezoelectric diaphragm within the case.
Preferably, the viscosity of the first adhesive before curing is greater than that of the second adhesive such that the first adhesive is more difficult to spread.
That is, when the first adhesive spreads easily because of low viscosity in the uncured state, the first adhesive may clog up the electrode of the piezoelectric diaphragm and the internal connections of the terminal electrodes, such that it is difficult to electrically connect the electrode of the piezoelectric diaphragm to the internal connections of the terminal electrodes when applying the conductive adhesives. Further, if the viscosity of the first adhesive is low, the first adhesive may not remain in the shortest connecting route between the external periphery of the piezoelectric diaphragm and the internal connections of the terminal electrodes. Thus, by using a first adhesive having high viscosity, such problems are solved, such that the piezoelectric diaphragm and the terminal electrodes are securely connected together with the conductive adhesives by detouring from the shortest route.
Preferably, the first adhesive is partially applied to vicinities of four corners of the piezoelectric diaphragm.
When using a thermosetting adhesive as the first adhesive, the deformation increases toward the center of four sides of the case, and the stress applied to the piezoelectric diaphragm is primarily exerted on the central portions of the four sides.
Whereas, when the first adhesive is partially applied to vicinities of four corners of the piezoelectric diaphragm, the deformation of the case during the curing of the first adhesive is greatly reduced, which substantially eliminates the effect on the piezoelectric diaphragm.
Preferably, the conductive adhesive is applied to vicinities of at least two of four corners of the piezoelectric diaphragm.
When the first adhesive is partially applied to vicinities of four corners of the piezoelectric diaphragm as described above, the strain in the piezoelectric diaphragm is greatly reduced. Moreover, when conductive adhesive is applied to vicinities of at least two of four corners of the piezoelectric diaphragm, the influence of the strain caused by the curing and contraction of the conductive adhesive is further reduced.
In the diaphragm, bending vibration in a length-bending mode and bending vibration in an area-bending mode is produced, corresponding to the manner of supporting the diaphragm. The former is a mode of bending-vibration in the plate-thickness direction when both lateral ends are supported and the latter is a mode in which the entire area of the diaphragm flexually vibrates in the plate-thickness direction when the four sides or the four corners are supported such that a position along the two diagonal lines on the principal surface of the diaphragm has the maximum displacement, i.e., the intersection of the diagonal lines has the maximum displacement.
According to the present invention, a urethane conductive paste is preferably used as the conductive adhesive. As the first adhesive, a material having a Young""s modulus after curing that is less than that of the conductive adhesive is preferably used, such as a urethane adhesive. As the second adhesive, a material having a Young""s modulus that is less than that of the first adhesive and having small curing contraction stress is preferably used, such as a silicone adhesive.
In addition, as the first and second adhesives, a cold-setting adhesive may alternatively be used. However, when applying it with a dispenser, because curing begins during coating, the dispenser is liable to clog up, which reduces work efficiency. Whereas the thermosetting adhesive has an advantage that the viscosity does not change during coating such that the dispenser does not clog up, which greatly improves work efficiency, because of its constant viscosity at the room temperature.
Other feature, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.